Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the integrity and fitness of the structure. In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies and structures with contoured surfaces. These composite structures, and a shift toward lightweight composite and bonded materials such as using graphite materials, dictate that devices and processes are available to ensure structural integrity, production quality, and life-cycle support for safe and reliable use. As such, it is frequently desirable to inspect structures to identify any defects, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. For example, typical defects in composite sandwich structures, generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, include disbonds which occur at the interfaces between the core and the skin or between the core and a buried septum.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors are typically used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure is commonly performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. While solid laminates and some composite structures are commonly inspected using one-sided pulse echo ultrasonic (PEU) testing, composite sandwich structures are commonly inspected using through-transmission ultrasonic (TTU) testing for high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and a receiver sensor, are positioned facing the other but contacting opposite sides of the structure. An ultrasonic signal is transmitted by at least one transducer, propagated through the structure, and received by the other transducer. Data acquired by sensors is typically processed and then presented to a user via a display as a graph of amplitude of the received signal. To increase the rate at which the inspection of a structure is conducted, a scanning system may include arrays of inspection sensors, i.e., arrays of transmitters and/or detectors. As such, the inspection of the structure can proceed more rapidly and efficiently, thereby reducing the costs associated with the inspection. However, it has traditionally not always been possible to perform continuous scanning of a structure with holes and off the edges of the structure. For example, inspection probes which contact and ride along the surface of the structure under inspection and are typically supported against the structure by the pull of gravity or by pressure exerted by a motion control system, referred to as part-riding probes, may fall through a hole in a structure or off the edge of the structure. Although a structure can be inspected in a manner to scan around holes, a second inspection method typically must be performed for inspecting the edges of the structure and edges defining holes in the structure. For example, a technician can manually scan around the edges of the structure and the edges of holes in a structure using a pulse-echo or through transmission ultrasonic hand probe.
Non-destructive inspection may be performed manually by technicians who typically move an appropriate sensor over the structure. Manual scanning requires a trained technician to move the sensor over all portions of the structure needing inspection. While manual scanning may be required around the edges of the structure and the edges of holes in a structure, manual scanning may also be employed for scanning the remainder of the structure.
Semi-automated inspection systems have been developed to overcome some of the shortcomings with manual inspection techniques. For example, the Mobile Automated Scanner (MAUS®) system is a mobile scanning system that generally employs a fixed frame and one or more automated scanning heads typically adapted for ultrasonic inspection. A MAUS system may be used with pulse-echo, shear wave, and through-transmission sensors. The fixed frame may be attached to a surface of a structure to be inspected by vacuum suction cups, magnets, or like affixation methods. Smaller MAUS systems may be portable units manually moved over the surface of a structure by a technician. However, for through-transmission ultrasonic inspection, a semi-automated inspection system requires access to both sides or surfaces of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for semi-automated systems that use a fixed frame for control of automated scan heads.
Automated inspection systems have also been developed to overcome the myriad of shortcomings with manual inspection techniques. For single sided inspection methods, such as pulse echo ultrasonic inspection, a single-arm robotic device, such as an R-2000iA™ series six-axis robot from FANUC Robotics of Rochester Hills, Mich., or an IRB 6600 robot from ABB Ltd. of Zurich, Switzerland, may be used to position and move a pulse echo ultrasonic inspection device. For through transmission inspection, a device such as the Automated Ultrasonic Scanning System (AUSS®) system may be used. The AUSS system has two robotically controlled probe arms that can be positioned proximate the opposed surfaces of the structure undergoing inspection with one probe arm moving an ultrasonic transmitter along one surface of the structure, and the other probe arm correspondingly moving an ultrasonic receiver along the opposed surface of the structure. Conventional automated scanning systems, such as the AUSS-X system, therefore require access to both sides or surfaces of a structure for through transmission inspection which, at least in some circumstances, will be problematic, if not impossible, particularly for very large or small structures. To maintain the transmitter and receiver in proper alignment and spacing with one another and with the structure undergoing inspection, the AUSS-X system has a complex positioning system that provides motion control in ten axes. The AUSS system can also perform pulse echo inspections, and simultaneous dual frequency inspections. It is often complicated, time consuming, and expensive to program and control an automated motion control system with respect to surface contours of a structure to within sufficient precision for performing inspection and maintaining consistent quality of inspection. The automated motion control system would have to make many small movements to keep an inspection probe riding on a contoured surface. For example, if the surface contour of a structure varies across a scanning area, an automated motion control system would need to both know the precise surface contour of the structure and be capable of moving an inspection probe over the surface of the structure while maintaining consistent orientation of the inspection probe with respect to the surface contours of the structure. The automated motion control system would have to be programmed for the shape, contours, and structural features of each structure to be inspected. Furthermore, variances in surface contours of the actual structure may not precisely match the data for the surface contours of the structure provided to the automated motion control system, thereby disturbing the automated motion control system's ability to maintain the inspection probe in consistent orientation with respect to the structure.
Accordingly, improved apparatus, systems, and methods for continuous inspection of structures with varying surface contours and features, such as large composite structures having surface curvature and including holes, and continuous inspection at edges of the structures are desired.